Physical implementation of a quantum computer presents a great challenge because quantum systems are susceptible to decoherence and because interactions between them cannot be controlled precisely. Quantum bits (a.k.a., qubits) must satisfy two basic requirements: they must preserve the quantum state intact for a sufficiently long time, and they must be easily operable. It has proved very difficult to meet both conditions together.
There have been impressive demonstrations of qubits using various kinds of systems, including Josephson junctions, yet building a full-scale computer remains a remote goal. In principle, scalability can be achieved by correcting errors at the logical level, but only if the physical error rate is sufficiently small. As an alternative solution, it has been observed that topologically ordered quantum systems are physical analogues of quantum error-correcting codes, and fault-tolerant quantum computation can be performed by braiding anyons.
Simpler examples of physical systems with error-correcting properties have been found. The key element of such systems, which may be referred to herein as 0-π qubit, is a two-terminal circuit built of Josephson junctions. Its energy has two equal minima when the superconducting phase difference between the terminals, θ=φ1−φ2, is equal to 0 or π. The quantum states associated with the minima, |0> and |1> can form quantum superpositions. It is essential that the energy difference between the two minima is exponentially small in the system size, even in the presence of various perturbations, hence the quantum superposition will remain unchanged for a long time. Implementations of some quantum gates have also been proposed.